Protective coating combination for lead frames

ABSTRACT

A lead frame for a semiconductor device includes a base which is coated by a protective coating. The lead frame base is made of a ferrous material. The protective coating is made by sequentially electroplating a copper-containing layer, a silver-containing layer and a palladium-containing layer. Protective coatings constructed in this way are bondable, solderable, oxidation resistant, corrosion resistant, free of lead (Pb), resistant to high temperatures, cost effective, and cosmetically acceptable. It is also possible to use a layer of tin or a tin alloy in place of the silver layer.

This is a divisional of application Ser. No. 08/513,690, filed Aug. 11,1995, now abandoned, which is a divisional application of applicationSer. No. 08/429,670 filed on Apr. 27, 1995, now U.S. Pat. No. 5,650,661,which is a continuation-in-part application of application Ser. No.08/174,890 filed on Dec. 27, 1993, now U.S. Pat. No. 5,436,082.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the construction of lead frames forsemiconductor devices.

2. Background Art

Copper lead frames are employed extensively in semiconductor devicepackages primarily due to their high thermal conductivity. Often, thelead frame is preplated with a suitable coating metal prior toencapsulation because the coating significantly reduces the number ofassembly steps typically required for post-assembly coating processes.The preplate coating protects the exposed lead frame, and promotes wirebonding and solderability of the leads after encapsulation.

Optimal lead frame coatings should be free of lead (Pb), corrosionresistant, oxidation resistant, resistant to high temperatures (over200° C.), wire-bondable, and solderable after assembly and burn-inprocesses. Additionally, the coating should be cost effective andcosmetically acceptable.

The prior art discloses lead frame coatings in which the outermostcoating layers comprise nickel and palladium. However, lead frames arerequired to possess a relatively high solderability coverage as measuredby standard solderability tests. In some cases, it has been found thatsuch preplated leads are not as solderable as might be desired.

Thus, there is a need for a lead frame structure which is readilysolderable, while still exhibiting the desirable characteristicsdescribed above.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a preplated copperlead frame coating that produces a finish that is readily bondable,solderable, free of lead, corrosion resistant and cosmeticallyacceptable. It is a further object of the invention to provide apreplated palladium lead frame coating finish that is undercoated withsilver and copper in order to improve solderability.

In accordance with the present invention a copper lead frame is firstcoated with nickel using methods well known in the art. Then a thinlayer of copper in the form of a strike coating is applied over thenickel. Next, a layer of silver is applied to the copper coating. Apalladium finish coat is then applied over the silver. The silver layeracts to promote solderability of the lead frame. The copper layerpromotes adherence between the silver layer and the nickel layer,improves solderability, and enhances tarnish resistance.

A better understanding of the features and advantages of the presentinvention will be obtained by reference to the following detaileddescription and accompanying drawings which set forth an illustrativeembodiment in which the principles of the invention are utilized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view (not to scale) illustrating a fragmentof an embodiment of a lead frame according to the present invention.

FIG. 2 is a cross-sectional view (not to scale) illustrating a fragmentof a second embodiment of a lead frame according to the presentinvention.

FIG. 3 is a cross-sectional view (not to scale) illustrating a fragmentof a third embodiment of a lead frame according to the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to a preferred embodiment of the invention shown in FIG. 1,the base layer 10 of the lead frame is made of any suitable lead framebase material, for example copper or a copper alloy. Two examples ofacceptable alloys are sold under the names Olin Alloy C-7025 and OlinAlloy 194, by Olin Brass of East Alton, Ill. One advantage of a copperbase metal layer is high thermal conductivity.

The base metal layer 10 is coated with a nickel layer 12. One method ofapplying the nickel layer 12 is by electrolytic deposition, a techniquewhich is well-known in the art. The thickness of nickel layer 12 shouldbe between 1.25 and 2.50 microns, preferably about 1.5 microns.

A copper layer 14 is applied over the nickel layer 12 by electrolyticdeposition. The copper layer 14 is made of either pure copper or acopper alloy, pure copper being preferred. Cu-Sn and Cu-Ag are twoexamples of copper alloys which exhibit sufficient solderability suchthat these alloys could be expected to work effectively as copper layer14 in embodiments of the present invention. The thickness of the copperlayer 14 should be between 0.08 and 0.38 microns, preferably about 0.25microns. One benefit of the copper layer 14 is improved solderability ofthe finished lead frame. The copper layer 14 also seems to enhancetarnish resistance of the lead frame coating.

A silver layer 16 is then applied over the copper layer 14 byelectrolytic deposition. The copper layer 14 promotes adherence betweenthe silver layer 16 and the nickel layer 12. The silver layer 16 is madeof either pure silver or a silver alloy, pure silver being preferred.Ag-Pd and Ag-Sn are two examples of materials which exhibit sufficientsolderability such that these alloys could be expected to workeffectively as the silver layer 16 in embodiments of the presentinvention. The thickness of the silver layer 16 should be between 0.13and 1.0 microns, preferably about 0.63 microns. One benefit of thesilver layer 16 is improved solderability of the finished lead frame.The silver layer 16 is also cost effective and wire-bondable.

A palladium layer 18 is applied over the silver layer 16, byelectrolytic deposition, to provide the coating surface finish. Thepalladium layer 18 is made of either pure palladium or a palladiumalloy, such as Pd-Ni or Pd-Co. The thickness of the palladium layer 18should be between 0.08 and 0.63 microns, preferably about 0.25 microns.The palladium layer 18 serves to protect the silver layer 16 fromtarnish. Furthermore, the palladium layer 18 of the present inventionimparts oxidation resistance, corrosion resistance and a cosmeticallyacceptable finish to the lead frame.

It has been found that the copper and silver layers 14 and 16 greatlyenhance the solderability performance of the palladium layer 18. Thesilver layer 16 dissolves more rapidly in solder than palladiummaterials. The copper layer 14 improves the solderability by alloyingwith the solder during the soldering process. For instance, the copperlayer 14 will alloy with Sn during soldering with Sn-Pb solder. Thecopper layer 14 is also thought to enhance wettability during thesoldering process. In this way, the combination of coating materialsdisclosed in the present invention exhibits improved solderability overpalladium and nickel coated lead frames.

Because lead frame coating materials, such as palladium, can beexpensive, another benefit of the present invention is that the copperlayer 14, the silver layer 16, and/or the palladium layer 18 can beselectively applied only in areas of the lead frame that are to besoldered after encapsulation. This selective application minimizes theamount of copper, silver and/or palladium which must be used.

It is to be understood that the present invention is not limited to theabove described embodiments, and that various changes and modificationscould be effected by one skilled in the art without departing from thespirit or scope of the invention as defined in the appended claims.

A description of some possible changes and modifications to theforegoing invention are discussed below.

While the lead frame base material is often made of copper or a copperalloy as described above, some lead frames employ ferrous, oriron-bearing alloy, base materials. If the lead frame has a ferrous basematerial, then it is possible to omit the nickel layer described above.Such a lead frame is shown in FIG. 2. The lead frame of FIG. 2 has aferrous base layer 100, a copper layer 140, a silver layer 160 and apalladium layer 180. The copper layer 140, silver layer 160 and thepalladium layer 180 are respectively similar to the copper layer 14, thesilver layer 16 and the palladium layer 18 discussed above.

While the nickel layer 12, discussed above, may be between 1.25 micronsand 1.50 microns, for some applications, the nickel layer may bethinner, down to 0.2 microns. The nickel layer 12 is generally, theleast ductile of all the layers. When choosing a type of nickel ornickel alloy for the nickel layer, it is preferable to choose moreductile alternatives in order to reduce cracking in the nickel layer.

For example, cracks in the nickel layer 12 may be caused by the formingoperation during plastic package assembly. The silver layer 16 and/orthe copper layer can help prevent these cracks from spreading to theextent that failure or a breach in the protective coating occurs.

Alloys of nickel may be used for the nickel layer 12. Possibly suitablenickel alloys include nickel-cobalt (Ni-Co), nickel-phosphorous (Ni-P),nickel-silver (Ni-Ag) and tin-nickel (Sn-Ni).

Instead of a nickel layer 12 (see FIG. 1), a palladium alloy layer 12may be used. Alternatives such as Pd-Sn (palladium -tin) and Pd-Co(palladium-cobalt) are possibilities for palladium alloy layer 12. Thepalladium alloy layer 12 must have a sufficient composition andthickness to protect the base layer 10 from oxidation and corrosion. Thepalladium alloy layer 12 should generally be between 0.1 and 2.5 micronsin thickness, preferably toward the thinner end of this range.

The layer 12 may also be a tin-cobalt (Sn-Co) layer 12 or pure cobalt(Co) layer 12. These are alternatives are also thought to provide thebase layer 10 with sufficient protection from oxidation and corrosion.From all the possible alternatives for layer 12, it is desirable tochoose a material which is low in cost and relatively less susceptibleto cracking.

The copper layer 14 (see FIG. 1), in addition to improving solderabilityand tarnish resistance of the lead frame, also helps to inhibitspreading of cracks which may occur in the less ductile nickel layer 12.While the copper layer 14 may be between 0.08 and 0.38 microns inthickness, the copper layer 14 may be as thick as 1.5 microns.Generally, as the copper layer 14 is made thicker, it will moreeffectively inhibit the spreading of cracks.

The silver layer 16 (see FIG. 1) also helps inhibit the spread of cracksin the less ductile nickel layer 12. Generally the thicker the aggregatethickness of the copper layer 14 and the silver layer 16, the moreeffectively these layers will inhibit the spreading of cracks.

Instead of a silver layer 16 (see FIG. 16), a tin layer 16 may be used.The tin layer 16 can be made of pure tin or a tin alloy, such as Sn-Pb(tin-lead). Preferably, the tin layer 16 should have a high meltingpoint, for instance above 150° C., so that it can withstand hightemperature fabrication or assembly operations. For example, possibletin alloys include tin-zinc (Sn-Zn) or tin-lead (Sn-Pb). The tin layer16 should be chosen to be easily solderable, in order to enhancesolderability of the finished lead frame. If the tin layer 16 is made ofpure tin or Sn-Pb, then the tin layer 16 can be applied by electrolyticdeposition. The tin layer 16 should generally be between 0.5 and 5microns in thickness, preferably about 2 microns. When the copper layer14 is present, the tin layer 14 should preferably be at least 0.5microns thicker than the copper layer 14 in order to prevent copper fromdiffusing toward the surface of the lead frame.

When there is a tin layer (instead of a silver layer), it may bepossible to omit the copper layer altogether. This is because there maybe sufficient adhesion of the tin layer to the underlying nickel andsufficient solderability without the copper layer. Such a lead frame isshown in FIG. 3. The base layer 210 is coated by a nickel layer 220. Thenickel layer 220 is coated by a tin layer 260. The tin layer 260 iscoated by a palladium layer 280. The base layer 210, the nickel layer220 and the palladium layer 280 are similar to corresponding layersdiscussed above in connection with the other embodiments.

What is claimed is:
 1. A method of fabricating a protective coating fora lead frame comprising the steps of:supplying a lead frame baseconsisting essentially of iron or an iron alloy; electroplating acopper-containing layer over the lead frame base; electroplating asilver-containing layer over the copper-containing layer; andelectroplating a palladium-containing layer over the silver-containinglayer.